The present invention relates generally to the removal of hydrogen sulfide from gas streams by contacting such streams with a regenerable, aqueous washing solution. More particularly, the present invention relates to the removal of sulfur particles from the washing solutions of hydrogen sulfide removal processes wherein absorbed hydrogen sulfide is converted to elemental sulfur.
With the increasing need for society and industry to coexist in close proximity to one another, industry is asked to produce increasingly pollution-free products in an increasingly non-polluting manner. A particularly nettlesome problem in this regard is that posed by the need to isolate and dispose of the toxic and highly odoriferous gas, hydrogen sulfide, a pollutant found in gas mixtures produced by a variety of important industrial operations (e.g., petroleum production and refining, the destructive distillation of coal, and oil shale, the gasification and liquefaction of coal, the generation of electrical power from geothermal steam, etc.).
Several processes have been developed in an effort to solve this problem. Among the most attractive of these processes are those which employ a regenerable, aqueous washing solution capable of absorbing hydrogen sulfide from an industrial gas stream and converting that hydrogen sulfide to elemental sulfur, a material which is not only relatively innocuous to the environment but also easily stored and valuable as a chemical raw material. Processes of this type are exemplified by the Stretford process wherein a pentavalent vanadium-containing washing solution is used to contact a sour gas stream and absorb hydrogen sulfide therefrom, and wherein the absorbed hydrogen sulfide is oxidized to elemental sulfur by the overall reaction: EQU H.sub.2 S+2VO.sub.3.sup.- .fwdarw.V.sub.2 O.sub.5.sup.= +S+H.sub.2 O(1)
The washing solution is thereafter regenerated for recycle by the removal of the elemental sulfur and by the reoxidation of the vanadium, usually with air via the reaction: EQU 2V.sub.2 O.sub.5.sup.= +O.sub.2 .fwdarw.4 VO.sub.3.sup.- ( 2)
A problem which has plagued the operators of such hydrogen sulfide removal processes is the difficulty in efficiently removing elemental sulfur particles from the circulating washing solution. Inefficient sulfur particle removal causes an unwanted buildup in the concentration of such particles within the washing solution. This buildup leads to a variety of problems. First of all, an increase in sulfur particle concentration increases the mass of the washing solution, causing an increase in pumping expenses. Also, as the particle concentration increases, some of the particles fall out of the washing solution and deposit within the circulating equipment. This deposition causes partial plugging of the circulating equipment, thereby further increasing pumping costs and often forcing a shutdown of operations. Furthermore, large quantities of circulating sulfur particles cause significant problems with corrosion and erosion of the circulating equipment. Finally, high concentrations of circulating sulfur particles cause accelerated chemical degradation of the washing solution since elemental sulfur reacts with various washing solution constituents to form nonregenerable contaminant salts.
It is known and is commonly practiced among designers and operators of hydrogen sulfide removal processes of the type described above to attempt the removal of elemental sulfur particles from the washing solution by circulating the washing solution through a tank-like oxidizer vessel wherein air is bubbled up from the bottom. With fresh, unused washing solutions, the attempt is fairly successful. The elemental sulfur particles, which when initially formed are about 1 to 5 microns in size, agglomerate to form sulfur clumps of 10 to 150 microns. These clumps are buoyed by the air bubbles to the surface of the liquid where they interact to form a sulfur-rich froth which rises above the liquid surface toward the top of the vessel. Near the top of the vessel the froth overflows through a weir-like opening and is collected in a tank-like sulfur froth collection vessel. From this collection vessel the sulfur-rich material is pumped to a sulfur separating device such as a filter, a centrifuge or an autoclave wherein the remaining amounts of entrained washing solution are separated from the elemental sulfur product.
After a washing solution has been in use for a period of time, however, the attempted removal of sulfur particles with air bubbles alone is often less than totally successful. This is believed to result from the buildup in used washing solutions of contaminant salts such as sulfates, thiosulfates, colloidal metal oxysulfides, and, in Stretford process systems, anthraquinone disulfonic acid oxidation products. These salts are believed to electrochemically inhibit the 1 to 5 micron sulfur particles from agglomerating into larger sulfur clumps. The 1 to 5 micron particles are not easily buoyed to the surface by bubbled air and so tend to stay suspended in the washing solution. The contaminant salts are also believed to electrochemically inhibit those sulfur particles which do find their way to the surface from interacting to form a sulfur-rich froth. Instead of forming a froth, the particles often form a large-bubbled foam which is relatively high in air and liquid content and relatively low in sulfur content. The presence of 1 to 5 micron sulfur particles exacerbates the foaming tendency of an older washing solution, as does the inadvertent introduction into the washing solution of small quantities of light hydrocarbons. Foam is a very poor vehicle for transferring sulfur out of the oxidizer vessel. Hence an inordinate quantity of the sulfur particles is not removed from the system but continue to circulate with the washing solution.
There is, therefore, a need for more efficient techniques for the removal of sulfur particles from the regenerable aqueous washing solutions of those hydrogen sulfide removal processes wherein hydrogen sulfide is converted into elemental sulfur.
Consequently, a primary object of this invention is to provide a novel and superior method for removing elemental sulfur particles from such washing solutions.
Another object of this invention is to provide a novel and superior method for creating and maintaining a stable, sulfur-rich froth above oxidizer vessels employed in such hydrogen sulfide removal processes.
A further object of this invention is to provide a novel and superior method for minimizing the foaming tendency of such aqueous washing solutions.
A still further object of this invention is to provide a novel and superior method for agglomerating fine sulfur particles within such aqueous washing solutions.
A still further object of this invention is to provide a novel and superior method for reducing the concentration of elemental sulfur particles in such circulating aqueous washing solutions and for maintaining such concentrations at low levels.
A still further object of this invention is to provide a novel and superior method for removing deposits of elemental sulfur from the internals of processing equipment used in such hydrogen sulfide removal processes such as pumps, lines, vessels, absorbers, etc.